This invention relates generally to heat insulating materials and structures for heating furnaces. More particularly, the invention relates to insulating materials and structures (hereinafter referred to generally as "insulating materials") suitable for use in heating furnaces in which nonoxidizing atmosphere such as a vacuum, an inert gas, or a reducing gas is used for purposes such as heat treatment of metals such as hardening, annealing, and brazing of metals, sintering of powdered alloys, evaporation deposition of metals, refining of electrolytic alumina, and melting of quartz.
The affixing of insulating materials on the inner wall surfaces of high-temperature heating furnaces for the purpose of maintaining the furnace interior temperatures is being practiced. As insulating materials for this purpose, materials such as bricks of graphite powder or alumina have heretofore been used, but it is difficult with these materials to obtain uniform thermal insulating property. Particularly in the case of a heating furnace to operate with a nonoxidizing atmosphere which requires a construction for isolating that atmosphere from the outside air, this work of installing the insulating material is very complicated and troublesome. Because of this problem, there has been a trend in recent years toward wide use, as insulating materials, of inorganic fibers of excellent heat resistance such as carbon fibers, ceramic fibers, slag fibers, and rock wool which have been formed into a bulky felt.
Inorganic fiber felts possess flexibility and are available in forms of almost uniform thickness. For these reasons, these felts are widely recognized as having useful features such as facility in securing to the inner wall surface of furnaces and in obtaining positive heat insulation effect, and short times for temperature rise and cooling due to their bulkiness and small heat capacity. However, it has been found that these inorganic fiber felts are accompanied by a number of problems in practical use which still require solutions.
More specifically, for persons concerned with enlarging as much as possible the uniform heating zones of heating furnaces thereby to elevate their capacities to process the materials being processed, there have been three typical points relative to which improvement in felt insulating materials has been desired, namely, (1) causing the felt to possess self-standing property, (2) eliminating fluff or nap of the felt, and (3) imparting tightness to the outer surfaces of the felt.
The lack of self-standing property in a felt necessitates the installation of a large number of supporting structures within the furnace in order to hold the felt in installed state at the inner surface of the furnace wall. This gives rise to a reduction of the processing space within the furnace and variations in the thickness and density of the felt due to the affixing thereof to the supporting structures, whereby the effective uniform heating zone within the processing space is reduced. This has been a source of dissatisfaction among persons concerned. The formation of fluff must be prevented because its scattering will become a cause of contamination of the material being processed.
Furthermore, tightness of the outer surfaces of the felt is interrelated to the serviceable life of the insulating material. More specifically, particularly in the case where a vacuum melting furnace or a vacuum evaporation deposition furnace is used, the material being melted sometimes undergoes bumping and, being scattered around the periphery of the crucible, adheres to the insulating material, or vaporized metal settles on the insulating material in some cases. However, the scattered material not only adheres in this manner to the surface of the felt but infiltrates through the gaps between the fibers and penetrates even into the inner layers of the felt. Parts of the felt to which the process material thus adheres undergo a remarkable decrease in mechanical strength and thereby readily fall off upon being subjected to impact or abrasion.
This vulnerability of the felt gives rise to results which are undesirable from the viewpoint of practical use of the felt as an insulating material, such as a great reduction the serviceable life thereof and a disturbance of the heat insulating property which causes temperature irregularities.
In order to overcome these difficulties accompanying the inorganic fiber felts known heretofore, one of the inventors of this invention has previously invented a process for producing a formed insulating material by impregnating a carbon fiber felt with thermosetting resin which is carbonizable and carbonizing this resin after modling and setting. By this process (as disclosed in Japanese Patent Publication No. 35930/1975) there is obtained an insulating material in which scattering of fluff of the carbon fiber felt is prevented, and the felt is self-supporting.
As a result of research carried out by the inventors of this invention, however, it has been found that the above described formed insulating material of impregnated carbon fiber felt is still accompanied by a number of problems.
More specifically, a formed insulating material obtained in the above described manner still has a bulk density of the order of only 0.11 to 0.13 grams/cm.sup.3., and while it can be said to be self-supporting, its prevention of release of fluff and improvement of the surface-tightness are inadequate. For this reason, the surface of the formed insulating material is worn away by contact and abrasion when the process material is being charged into or taken out of the furnace or by erosion caused by high-velocity gas flow due to the flowing out and in of ordinary nonoxidizing gas such as displacement and discharge of gas within the furnace or introducing of cooling gas carried out during the operation of a vacuum furnace. To date there has been no formed insulating material in which this erosion and scattering of the felt fluff can be prevented.
Furthermore, since the surface-tightness of this molded insulating material is inadequate, the deterioration of the insulating material due to scattering, adherence, and penetration of the process material has been almost unavoidable. This problem can be overcome to some extent by increasing the quantity of the carbonaceous binder to increase the tightness of the felt surface. However, the carbonaceous binder, which has been obtained by carbonization of a thermosetting resin, has, itself, low strength, is fragile, and is readily pulverized, and even if scattering of fluff can be prevented, there is the possibility of increase in the scattering of powder of the binder itself. Still another problem is that, in order to increase the quantity of the binder, it is necessary to impregnate the felt with a very large quantity of the resin which is the precursor of the binder, but in this case also, at the time of carbonization after forming, a volumetric shrinkage corresponding to the carbonization rate of that resin occurs, and the shape of the entire formed insulating material becomes deformed.
That is, according to the results of our study, it can be said that the thermally insulating characteristic and the above mentioned three requirements of self-standing property, prevention of napping or fluffing, and surface-tightness cannot all be satisfied by a single layer of insulating material.
As a result of our further research based on this knowledge, we have observed that a graphite sheet has excellent surface-tightness and, at the same time, has the capability of being impregnated with the above described carbonaceous binder or its precursor, the carbonizable resin. We have found further that, accordingly: a graphite sheet can be strongly bonded to a carbon fiber felt by a carbonaceous binder of this character; the multilayer insulating material obtained in this manner fully satisfies the above stated required characteristics of an insulating material; and, by bonding a graphite sheet on the surface, radiation heat is shielded off, and the adiabatic efficiency is raised by approximately 20 percent (i.e., the required heating energy is reduced by approximately 20 percent).